The broad goal of the proposed studies is to understand how a nuclear hormone receptor (NHR) regulates a developmental pathway and how it becomes localized to the plasma membrane in response to an extracellular signal. In addition to their nuclear functions in transcriptional regulation, NHRs have been shown to perform distinct membrane-associated signaling activities. We have identified the first NHR, C. elegans DPR-1, that relocalizes primarily to the plasma membrane. This relocalization is triggered by dauer pheromone, which activates formation of the alternative "dauer larva" form of the animal. Genes that function early, but not later, in the pathway regulating dauer formation, affect DPR-1 localization. A deletion mutation of dpr-1 strongly reduces response to pheromone. Taken together, these results lead us to hypothesize that DPR-1 may regulate dauer development at the level of dauer pheromone reception, and that it may do so through a membrane-based signaling system. I propose the following aims: Aim 1: Characterize the genetic relationships between DPR-1 and other components of the known dauer regulatory pathway, and test its possible action with other NHRs that may collaborate redundantly with DPR-1 to mediate its role in dauer pheromone responsiveness. Aim 2: Analyze the kinetic and qualitative response of DPR-1 subcellular localization to dauer pheromone components, examine the possible direct action of pheromone on its localization in heterologous cell types, and assess whether it activates guanylyl cyclase activity in such cells. Aim 3: Analyze DPR-1 domains required for relocalization and activation of dauer development and examine whether its membrane localization is necessary or sufficient for its dauer- regulating activity. Steroids and other lipophilic hormones that are ligands for NHRs are pivotal regulators of diverse biological processes, including reproduction and tumor formation. Many studies have revealed the importance of non-genomic NHR functions in these processes. A more complete understanding of non- genomic NHR function will provide further insights into mechanisms of action of these key hormonal regulatory systems.